A few thoughts, having heard both, but not in direct comparison and a couple of years apart.
They are both more detailed and analytical than your LS50s, of which I'm a big fan. I think their compromises are musical ones, which are easy to tolerate. I agree with those who suggest also looking at higher end KEF. Ref 1s sound much bigger than they have any right to; possibly even than the PMCs.
Of the two, the ATC is more analytical, and would require a better standard of amplifier to get the best from it. Though both PMC and ATC are studio orientated, ATC is closer to what I'd call studio sound. The twenty 26 is more living room friendly to my eyes, and should tolerate slightly more compromised positioning.
If I were wanting a very good set up, with say a decent Naim streaming system, I know the PMCs would do the job. If I wanted to review recordings for a living, I might choose the ATC knowing it wouldn't make mediocre recordings sound good.
I hope this gives you a few subjective pointers. Interested in others views.